The present invention relates to an electric power steering device having an electric motor for producing assisting steering power.
An extremely large manual force is required for changing the direction of wheels of a vehicle by rotating a steering wheel when the vehicle is not running or running at a low speed. This problem is serious particularly in so-called FF vehicles (front engine front-wheel drive vehicles) which are now becoming popular, because in such vehicles a large weight is applied to the front wheels.
Power steering devices are known as devices for assisting the driver in maneuvering the steering wheel. Briefly, such a device provides a driving force in response to the manual steering force exerted by the driver and transmits the driving force to the steering device. Although various power steering devices have been put into practical use up to now, most of such power steering devices are of the hydraulic type. The hydraulic type power steering device has parts such as a control valve, hydraulic cylinders and so forth, and is adapted to produce an assisting steering power by displacing working oil in accordance with the steering force.
The hydraulic power steering devices, however, suffer from the following disadvantages. Namely, the constituents such as the control valve, the hydraulic cylinder and so forth are generally large in size. The pipes through which fluid is supplied to these constituents have to be bent at a radius of curvature greater than a predetermined radius in order to prevent a large pressure drop for the oil. Furthermore, it is necessary to incorporate seals for preventing leaking of oil. In addition, it is very difficult to handle the device as a whole during mounting. For these reasons, the hydraulic type power steering device is not suited to vehicles having only a small mounting space as in the case of the FF vehicles.
FIG. 1 shows a relationship between the power required for the steering and the vehicle speed. It will be understood that a large steering power is required when the vehicle speed is low and, as the vehicle speed is increased, the power required for the steering is decreased gradually. Therefore, in the conventional power steering devices in which a constant effect is produced regardless of the vehicle speed, the steering device becomes too sensitive to the manual steering force when the vehicle is running at a high speed, although a moderate assisting effect is obtained when the vehicle speed is low. This may cause a danger that the driver who is not familiar with the power steering device applies too large a manual force on the steering wheel. Even the drivers who are well trained in power steering driving prefer to be required to exert a large maneuvering force during high-speed running of the vehicle. To cope with this demand, a power steering device has been proposed in which the assisting steering power is changed in accordance with a change in the vehicle speed. Such an improved power steering device, however, requires a highly complicated steering device resulting in a raised cost of production.